Method and apparatus for post-processing of episodes detected by a medical device

ABSTRACT

A method and system for determining undersensing during post-processing of sensing data generated by a medical device that includes transmitting a plurality of stored sensing data generated by the medical device to an access device, the stored sensing data including sensed atrial events and sensed ventricular events. The access device determines, in response to the transmitted data, instances where the medical device identified a cardiac event being detected in response to the sensing data, and determines whether one of a predetermined number of undersensing criteria have been met in response to the transmitted data.

CROSS REFERENCE TO RELATED APPLICATION

This is a utility application of and claims priority to provisionalapplication U.S. Ser. No. 60/740,219 filed Nov. 28, 2005, entitledMETHOD AND APPARATUS FOR POST-PROCESSING OF EPISODES DETECTED BY AMEDICAL DEVICE, incorporated herein by reference in its entirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

Cross-reference is hereby made to the commonly assigned related U.S.applications, attorney docket number P22894.01, entitled “METHOD ANDAPPARATUS FOR POST-PROCESSING OF EPISODES DETECTED BY A MEDICAL DEVICE”,to Gunderson et al.; attorney docket number P22894.03, entitled “METHODAND APPARATUS FOR POST-PROCESSING OF EPISODES DETECTED BY A MEDICALDEVICE”, to Gunderson et al.; attorney docket number P22894.04, entitled“METHOD AND APPARATUS FOR POST-PROCESSING OF EPISODES DETECTED BY AMEDICAL DEVICE”, to Gunderson et al.; attorney docket number P22894.05,entitled “METHOD AND APPARATUS FOR POST-PROCESSING OF EPISODES DETECTEDBY A MEDICAL DEVICE”, to Gunderson et al.; attorney docket numberP22894.06, entitled “METHOD AND APPARATUS FOR POST-PROCESSING OFEPISODES DETECTED BY A MEDICAL DEVICE”, to Gunderson et al.; attorneydocket number P22894.08, entitled “METHOD AND APPARATUS FORPOST-PROCESSING OF EPISODES DETECTED BY A MEDICAL DEVICE”, to Gundersonet al.; all filed concurrently herewith and incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to medical devices, and moreparticularly to a method and apparatus for improved post-processingevaluation of episodes detected by a medical device.

BACKGROUND OF THE INVENTION

As memory and diagnostic capacity in implantable medical devices, suchas implantable cardioverter-defibrillators (ICDs), for example,increase, the amount of time required to adequately review the availabledata associated to determine whether the detection of episodes anddelivery of therapy by the device was appropriate also increases. Sincethe number of identified ICD indications continues to increase, whilethe amount of time that is available for post-process review of datadecreases, the classification of ICD episodes requires significantlevels of expertise. As a result, the number of clinicians having therequired expertise has been reduced, which could result in a reductionin the quality of management of those patients having implanted devices.Therefore, an algorithm that post-processes and automatically reviewseach previously detected episode upon interrogation could address theseconcerns by accurately classifying episodes and potentially suggestingICD parameter changes and/or medical therapy, such as changes inmedication, therapy delivery, use of ablation procedures, etc.

Reviewing the data stored in the ICD memory at clinic follow-up requiresexpert knowledge to discriminate between true ventricular arrhythmiasand unnecessary detection of non-ventricular arrhythmias. As the ICDpopulation increases, the time needed to review all ICD detectedepisodes with careful detail also increases. Automatically identifyingICD stored events that may have been inappropriately detected asepisodes by the device may decrease the time required to review episodesand to assure that unnecessary detections are properly reviewed.

Therefore, an algorithm that correctly classifies each detected episodeduring post-processing review of data stored in an implantable device isneeded in order to reduce the clinician time to review episodes, and togive the clinician confidence that each incorrect ICD detection wasbrought to their attention.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and features of the present invention will be appreciated as thesame becomes better understood by reference to the following detaileddescription of the embodiments of the invention when considered inconnection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an exemplary implantable medical devicesystem for classifying a cardiac event according to an embodiment of thepresent invention;

FIG. 2 is a schematic diagram of a system for classifying an eventaccording to an embodiment of the present invention;

FIG. 3 is a flowchart of classifying of a cardiac event according to anembodiment of the present invention;

FIG. 4 is a graphical illustration of a determination of an A/V ratioduring re-classification of a detected event according to an embodimentof the present invention;

FIG. 5 is a graphical representation of determining whether there was anabrupt onset associated with a detected event according to an embodimentof the present invention;

FIGS. 6A-6C are graphical representations of determining whether thereis a leading chamber according to an embodiment of the presentinvention;

FIG. 7 is a graphical illustration of generation of a template accordingto an embodiment of the present invention;

FIG. 8 is a graphical representation of exemplary data utilized in theclassification of an event by an implantable medical device;

FIG. 9 is a flowchart of a method of determining whether data previouslyutilized by a device to identify a cardiac event corresponds to regularsensing of events by the device according to an embodiment of thepresent invention;

FIG. 10 is a flowchart of a method of determining whether datapreviously utilized by a device to identify a cardiac event correspondsto regular sensing of events by the device according to an embodiment ofthe present invention; and

FIG. 11 is a graphical illustration of determining whether undersensinghas occurred according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of an exemplary implantable medical devicesystem for classifying a cardiac event according to an embodiment of thepresent invention. As illustrated in FIG. 1, a system for classifyingevents according to an embodiment of the present invention includes animplantable medical device 220, such as implantable cardiac pacemaker,implantable cardioverter/defibrillator (ICD), orpacemaker/cardioverter/defibrillator, for example, electrically andmechanically attached to one or more leads 114 placed within the heartof the human body 105 that is used to acquire and process physiologicaldata from the patient and to deliver therapy in response to the acquireddata. The system for classifying events according to an embodiment ofthe present invention may also be in other medical devices, such as acardiomyostimulator, a drug delivery system, cardiac and otherphysiologic monitors, electrical stimulators including nerve, muscle,and deep brain stimulators, cochlear implants, and heart assist IMDs orpumps, etc.

The data acquired by the implantable medical device 220 can be monitoredby an external system, such as the access device 240, comprising aprogramming head 122, which remotely communicates with the implantablemedical device 220. The programming head 122 is utilized in accordancewith medical device programming systems known to those skilled in theart having the benefit of the present disclosure, for facilitatingtwo-way communication between the implantable medical device 220 (e.g.,pacemaker) and the access device 240. In this way, the classification ofcardiac events according to an embodiment of the present invention maytake place in the access device 240 once the required data istransmitted from the medical device 220 to the access device 240. Accessdevice 240 may be located at a remote location relative to the patientand therefore the classification of events according to an embodiment ofthe present invention may be performed in the access device 240 once therequired data has been transmitted from the medical device 220 to theaccess device 220 via the internet, for example.

The implantable medical device 220 is housed within a hermeticallysealed, biologically inert outer canister or housing 113, which mayitself be conductive so as to serve as an electrode in the implantablemedical device 220 pacing/sensing circuit. One or more leads,collectively identified with reference numeral 114 in FIG. 1, areelectrically coupled to the implantable medical device 220 and extendinto the patient's heart 116 via a vein 118. Disposed generally near adistal end of the leads 114 are one or more exposed conductiveelectrodes (sensor/therapy delivery device 210) for receiving electricalcardiac signals or delivering electrical pacing stimuli to the heart116. The leads 114 may be implanted with their distal end situated ineither the atrium or ventricle of the heart 116. In an alternativeembodiment, the sensor/therapy delivery device 210, or the leads 114associated with the sensor/therapy delivery device 210, may be situatedin a blood vessel on the heart 116, such as a vein 118.

FIG. 2 is a schematic diagram of a system for classifying an eventaccording to an embodiment of the present invention. As illustrated inFIG. 2, a system 200 for evaluating a prior classification of an eventincludes a sensor/therapy delivery unit 210, an implantable medicaldevice 220, and an access device 240. Embodiments of the presentinvention can provide one or more of a plurality of physiological dataand non-physiological data from the sensor/therapy delivery unit 210 tothe implantable medical device 220, which are then processed and storedin the implantable medical device 220. The sensor/therapy delivery unit210 may include a plurality of sensors that are capable of acquiringphysiological and non-physiological data. Based upon data from thesensor(s) 210 and other factors, the implantable medical device 220 maydeliver a therapy to a portion of the patient's body 105, via thesensor/therapy delivery unit 210. The access device 240 can be used tore-classify identification of events by the device 220, using themethods described below, and may reprogram and/or make modifications tothe operation of the implantable medical device 220 accordingly. Inanother embodiment of the present invention, the device 220 is used tore-classify prior identification of events by the device 220 using themethods described below.

FIG. 3 is a flowchart of classifying of a cardiac event according to anembodiment of the present invention. The present invention relates to amethod and apparatus for evaluating prior classifications of events bythe device 220 that includes determining instances where an event waspreviously classified by the device as a detected event Block 300, suchas ventricular fibrillation of ventricular tachycardia, for example. Adetermination is made as to whether the classification of the event as adetected event was made based on accurate sensing data. For example, adetermination is made as to whether data associated with atrial sensingduring the original classification of the event included the correctidentification of atrial depolarizations that are both regular andspontaneous, i.e., not induced, Block 302. A detailed description ofidentification of atrial depolarizations that are regular andspontaneous, according to an embodiment of the present invention, is setforth below in reference to FIGS. 9-11.

If the atrial sensing is not determined to be both spontaneous andregular, No in Block 302, the detected event is marked as being unknown,Block 301, and the next event that was previously identified by thedevice as a detected episode is identified, Block 300. Once the atrialsensing is determined to be both spontaneous and regular, Block 302, thedetected event is verified by being re-classified as either asupraventricular tachycardia event or a VT/VF event. According to anembodiment of the present invention, this re-classification of thedetected event is made by determining an A/V ratio associated with theratio of atrial sensed events to ventricular sensed events over apredetermined window of sensed events occurring prior to the detectionof the event, Block 304. For example, as illustrated in FIG. 4, apredetermined window 400 utilized for calculating the A/V ratio may bedefined by the last 12 ventricular sensed events V_(S) occurring justprior to the point of the detection of the event 402, although it isunderstood that the window 400 may be defined by any predeterminednumber of ventricular sensed events V_(S) prior to detection. The numberof atrial sensed events A_(S) occurring within the window 400 are thendetermined, and utilized to determine the A/V ratio. In the example ofFIG. 4, fourteen atrial sensed events A_(S) occur within the window 400.

If the number of atrial sensed events A_(S) occurring during the window400 is equal to the number of ventricular sensed events V_(S) andtherefore the A/V ratio is equal to one, Yes in Block 306, adetermination is made in FIG. 3 as to whether the atrial sensed eventsA_(S) are evenly distributed with the ventricular sensed events V_(S) ina one-to-one distribution, i.e., one A for each V), Block 308. Forexample, a determination is made as to whether there is only one atrialsensed event A_(S) located between each adjacent pair of all of theventricular sensed events V_(S) available in the detected episode priorto the window 400. If the atrial sensed events A_(S) are evenlydistributed with the ventricular sensed events V_(S) in a one-to-onedistribution, Yes in Block 308, i.e., there is one atrial sensed eventA_(S) located between each adjacent pair of all of the ventricularsensed events V_(S), the event is identified as being a supraventriculartachycardia event, Block 310. The rationale behind this decision is thata ventricular arrhythmia would have a shorter ventricular cycle lengththan the atrial cycle length at some point in the episode.

According to an embodiment of the present invention, during thedetermination as to whether the number of atrial sensed events A_(S)occurring during the window 400 is equal to the number of ventricularsensed events V_(S), the A/V ratio may be determined to be equal to one,Yes in Block 306, if the number of atrial sensed events is within apredetermined range of the number of ventricular sensed events, such asplus or minus one. In addition, during the determination in Block 308 asto whether the atrial sensed events A_(S) are evenly distributed withthe ventricular sensed events V_(S) in a one-to-one distribution, apredetermined number of instances where there are more than one atrialsensed event between adjacent pairs of ventricular sensed events may beincluded. For example, a determination is made as to whether there isonly one atrial sensed event A_(S) located between each adjacent pair ofthe ventricular sensed events V_(S) prior to the window 400 and lessthan a predetermined number of adjacent pairs of ventricular events,such as three, having two beats.

If the atrial sensed events A_(S) are determined to be not evenlydistributed with the ventricular sensed events V_(S) in a one-to-onedistribution, No in Block 308, a determination is made as to whetherthere was an abrupt onset associated with the detected event, Block 312.

FIG. 5 is a graphical representation of determining whether there was anabrupt onset associated with a detected event according to an embodimentof the present invention. As is known in the art, a cardiac event isdetected once a predetermined number of intervals, commonly referred toas the number of intervals to detection (NID), having an interval ratethat is less a predetermined detection rate have been detected. In theexemplary graphical representations of FIGS. 4 and 5, for example, thenumber of intervals to detection (NID) is 24 intervals. Therefore,during the initial detection process, the device 220 detects theoccurrence of a cardiac event once the detection criteria have been met,i.e., once 24 intervals having a rate less than the threshold rate aredetected.

As illustrated in FIG. 5, the determination of whether the onset of theevent is abrupt is made by first determining the median of apredetermined number of detected intervals 500, such as eight detectedintervals, for example, that occur just prior to an interval 502corresponding to when the event is detected by the device as a cardiacevent, along with the median of a predetermined number of detectedintervals 504 occurring prior to the NID intervals 506 associated withprior detection of the event by the device 220 (at the beginning of thestored episode, for example). A difference 508 between the median of thepredetermined number of detected intervals 500 and the median of thepredetermined number of detected intervals 504 is determine, and theonset of the event is determined to be abrupt if the difference 508between the medians is greater than a median difference threshold, suchas 60 milliseconds, for example.

According to an embodiment of the present invention, the determinationas to whether the onset of the event is an abrupt onset may be madeusing methods described in U.S. patent application Ser. No. 11/461,269,filed Mar. 29, 2006, to Stadler et al. incorporated herein by referencein it's entirety.

Returning to FIG. 3, if the difference 508 between the two medians isnot greater than the median difference threshold, and therefore theonset of the event is determined not to be an abrupt onset, No in Block312, the event is identified as being unable to be evaluated and theprocess continues with a next post-processing determination of theaccuracy of the classification of the event, such as Block 312 forexample, described below. If the difference 508 between the two mediansis greater than the median difference threshold, and therefore the onsetof the event is determined to be an abrupt onset, Yes in Block 312, theaccuracy of the classification of the event is further evaluated bydetermining whether the heart rhythm is being initiated by conduction ineither the ventricles or in the atrium. For example, according to anembodiment of the present invention, an onset threshold 510 isdetermined, Block 314, as the sum of the median of the predeterminednumber of detected intervals 500 occurring just prior to the interval502 associated with detection of the event and a portion of thedifference 508 between the two medians, set forth by the followingequation:Onset Threshold=episode median+X*median difference   Equation 1

where the episode median is the median of the predetermined number ofevents 500 occurring just prior to the interval 502 associated withdetection of the event, the median difference is the difference 508between the two medians, and X*median difference corresponds to aportion of the difference 508 between the two medians. According to oneembodiment of the present invention, X is equal to two thirds, so thatX*median difference corresponds to two thirds of the median difference.

Once the onset threshold 510 has been determined, Block 314, a spatialreference point is identified in Block 316 that is utilized to form awindow for determining whether conduction of the heart rhythm is beinginitiated by one of the atrial and the ventricular chambers. Forexample, according to an embodiment of the present invention, anRR-interval associated with pre-NID or sinus rhythm, i.e., greater thanthe onset threshold 510, occurring prior to the interval 502corresponding to when the event is determined to be detected, isidentified and utilized as a spatial reference point for forming thewindow.

Using the example of FIG. 5, starting with and working backward in timefrom the interval 502 corresponding to detection of the event, theidentification of the spatial reference point is made by determiningwhen a predetermined number of sequential adjacent intervals occurringprior to interval 502 are greater than the onset threshold 510. Forexample, according to an embodiment of the present invention, adetermination is made as to whether three intervals of five sequentialadjacent intervals 512 occurring just prior to interval 502 are greaterthan the onset threshold 510. If three intervals of the window of fivesequential adjacent intervals 512 (including interval 502) are notgreater than the onset threshold 510, a determination is made as towhether three intervals of the next window of five adjacent intervals514, working backward in time from the interval 502 corresponding todetection of the event, are greater than the onset threshold 510. Theprocess continues until an interval is identified where three intervalsof a window 516 of five intervals are greater than the onset threshold510.

Once window 516 is identified, the interval 518 of window 516 that isgreater than the onset threshold 510 and closest to the interval 502corresponding to detection of the event is set as the spatial referencepoint for forming the window for determining whether conduction of theheart rhythm is being initiated by one of the atrial and the ventricularchambers.

FIGS. 6A-6C are graphical representations of determining whether thereis a leading chamber according to an embodiment of the presentinvention. Using a predetermined number of intervals centered around thedetermined spatial reference point, i.e., interval 518, the numbersensed atrial events that occur between each interval is determined, theresults of which are then utilized to determine whether conduction ofthe heart rhythm is being initiated by one of the atrial chamber, Block318, and the ventricular chamber, Block 320. For example, as illustratedin FIGS. 6A and 6B, using three intervals centered around interval 518,a determination is made as to the number of atrial sensed events thatoccur between the resulting seven intervals.

If, as illustrated in FIG. 6A, there is one atrial sensed event A_(S)between each of the adjacent ventricular sensed events V_(S) forming theseven intervals, the atrium is determined to be initiating conduction,Yes in Block 318, and the event is identified as being asupraventricular tachycardia event, Block 310. If, as illustrated inFIG. 6B, there is one atrial sensed event A_(S) between each of theadjacent ventricular sensed events V_(S) for six of the seven intervalsand no atrial sensed event A_(S) between adjacent ventricular sensedevents V_(S) for one of the intervals, the ventricles are determined tobe initiating conduction, Yes in Block 320, and the event is classifiedas being a VT/VF event, Block 322. Finally, if, as illustrated in FIG.6C, there is one atrial sensed event A_(S) between each of the adjacentventricular sensed events V_(S) for less than six of the sevenintervals, and no atrial sensed event A_(S) between adjacent ventricularsensed events V_(S) for more than one of the intervals, then neither theatrium nor the ventricles are determined to be driving conduction, No inBlocks 318 and 320, and a determination is made as to whether theeffects of antitachycardia pacing are indicative of a supraventriculartachycardia event, Block 321 of FIG. 3.

According to an embodiment of the present invention, the determinationas to whether the effects of antitachycardia pacing are indicative of asupraventricular tachycardia event, Block 321, are made utilizing themethod of dynamic discrimination described in commonly assigned U.S.patent application Ser. No. 10/839,634, filed May 5, 2004, and entitled“DYNAMIC DISCRIMINATION UTILIZING ANTI-TACHY PACING THERAPY IN ANIMPLANTABLE MEDICAL DEVICE”, incorporated herein by reference in it'sentirety. For example, instances where the device 220 delivered anantitachycardia pcing regimen are identified, and the EGMs associatedwith the therapy are reviewed by determining a mean cycle length betweenatrial events occurring prior to the delivery of the antitachycardiapacing therapy and comparing the determined mean atrial cycle lengthwith an atrial cycle length during the delivery of the pacing therapy.If the difference between the mean atrial cycle and the atrial cyclelength during the delivery of the pacing therapy is less than or equalto a predetermined atrial cycle length threshold, such as 30 ms forexample, the event is re-classified as being a supraventriculartachycardia event, Block 310. If the difference between the mean atrialcycle length and the atrial cycle length during the delivery of thepacing therapy is greater than the predetermined atrial cycle lengththreshold, the minimum interval (during NID intervals associated withdetection of the event) and the maximum interval of all intervals areidentified and a determination is made as to whether the differencebetween the maximum interval and the minimum interval is greater than apredetermined threshold, such as 100 ms for example, Block 332.

If the difference between the maximum interval and the minimum intervalis not greater than a predetermined threshold, No in Block 332, themorphologies of each of the intervals associated with the detecting ofthe event are compared with the template or templates stored in Block326, and a determination is made for each as to whether the correlationof the morphology is greater than a predetermined correlation threshold,such as 0.95 for example. A determination is made as to whether thenumber of intervals that correlate with the stored threshold is greaterthan a predetermined matching percentage threshold, such as 80% forexample, Block 336.

If the difference between the maximum interval and the minimum intervalis greater than the predetermined threshold, Yes in Block 332, themorphology of the maximum interval is compared with the morphology ofthe minimum interval, and a determination is made as to whether thecorrelation of the maximum interval with the minimum interval is greaterthan a predetermined correlation threshold, Block 334, such as 0.94 forexample.

If the correlation of the maximum interval with the minimum interval isgreater than the predetermined correlation threshold, Yes in Block 334,the detected event is re-classified as being a supraventriculartachycardia, Block 310. If the correlation of the maximum interval withthe minimum interval is not greater than the predetermined correlationthreshold, No in Block 334, the morphologies of each of the intervalsassociated with the detecting of the event are compared with thetemplate or templates stored in Block 326, and a determination is madefor each as to whether the correlation of the morphology is greater thana predetermined correlation threshold, such as 0.95 for example. Adetermination is made as to whether the number of intervals thatcorrelate with the stored template is greater than a predeterminedmatching percentage threshold, Block 336, such as 80% for example.

According to an embodiment of the present invention, either a singletemplate or more than one template may be stored, such as four forexample, with each of the stored templates being utilized in there-classification of detected events as described below. In addition,the stored templates may be generated from any episode from the samepatient, either during the analysis of the current save to disk oracross different interrogations.

If the number of intervals, having morphologies that are determined tohave a correlation with the stored template that is greater than thecorrelation threshold, is greater than the predetermined matchingpercentage threshold, Yes in Block 336, the previously detected event isre-classified as being a ventricular tachycardia event, Block 338. Ifthe number of intervals having morphologies 80 that are determined tohave a correlation with the stored template that is greater than thecorrelation threshold is not greater than the predetermined matchingpercentage threshold, No in Block 336, a re-classification of thedetected event is not possible, and therefore the previously detectedevent is identified as being unknown, Block 340.

If the number of atrial sensed events A_(S) occurring during the window400 is not equal to the number of ventricular sensed events V_(S) andtherefore the A/V ratio is not equal to one, No in Block 306, adetermination is made in Block 324 as to whether the number ofventricular sensed events V_(S) is greater than the number of atrialsensed events A_(S) and therefore the A/V ratio is less than one. If thenumber of ventricular sensed events V_(S) is greater than the number ofatrial sensed events A_(S) and therefore the A/V rate is less than one,Yes in Block 324, a template is generated and stored, Block 326, and theevent is classified as a VT/VF event, Block 322.

FIG. 7 is a graphical illustration of generation of a template accordingto an embodiment of the present invention. As illustrated in FIG. 7,according to an embodiment of the present invention, in order togenerate the template in Block 328, the present invention correlativesthe morphology of adjacent intervals, starting with and working backwardin time from the interval 502 associated with detection of the event.For example, a morphology of the interval 502 is determined andidentified by a template, i.e., template A in FIG. 7. The morphology ofan interval 560 occurring just prior and adjacent to interval 502 iscompared with template A, and if the morphology of interval 560 has apredetermined correlation with the morphology of interval 502, such as acorrelation that is greater than or equal to 0.82, for example, then thetwo intervals are identified as being correlated and interval 560 isidentified by template A. If the morphologies do not correlate, i.e.,have a correlation less than 0.82, then the morphology of interval 560is identified by a new template, template B in FIG. 7.

Once the correlation of intervals 502 and 560 have been determined andthe morphologies are identified by a template, a determination is madeas to whether the morphology of a next interval 562, occurring justprior to the most recently correlated interval, interval 560, correlateswith one of the previously generated templates, i.e., template A ortemplate B. In particular, in the example of FIG. 7, since thecorrelation of interval 562 with template B is equal to 0.98 and thecorrelation of interval 562 with template A is equal to 0.72, interval562 is determined to correlate with template B (0.98 is greater than0.82) but not with template A (0.72 is not greater than or equal to0.82). As a result, the morphology of interval 562 is identified ashaving the same morphology as template B.

Once the correlation of interval 562 with the previously generatedtemplates is completed and interval 562 has been identified with atemplate, a determination is made as to whether the morphology of a nextinterval 564, occurring just prior to the most recently correlatedinterval, interval 562, correlates with one of the previously generatedtemplates, i.e., template A or template B. In particular, in the exampleof FIG. 7, since the correlation of interval 564 with template B isequal to 0.80 and the correlation of interval 564 with template A isequal to 0.81, interval 564 is determined not to correlate with templateB (0.80 is less than 0.82) or with template A (0.81 is less than 0.82).As a result, the morphology of interval 562 is identified as having adifferent morphology and is therefore a new template C is generatedbased on the morphology of interval 564 and the morphology of interval564 is identified as having the same morphology as new template C.

Once the correlation of interval 564 with the previously generatedtemplates is completed and interval 564 has been identified with atemplate, a determination is made as to whether the morphology of a nextinterval 566, occurring just prior to the most recently correlatedinterval, interval 565, correlates with one of the previously generatedtemplates, i.e., template A, template B or template C. In particular, inthe example of FIG. 7, since the correlation of interval 566 withtemplate C is equal to 0.82, the correlation of interval 566 withtemplate B is equal to 0.99 and the correlation of interval 566 withtemplate A is equal to 0.69, interval 566 is determined to be correlatedwith template C (0.82 is greater than or equal to 0.82) and template B(0.99 is greater than 0.82), but not with template A (0.69 is less than0.82). Since the morphology of interval 566 is identified as having thesame morphology as template B and template C, a determination is made aswhether the correlation of template interval 566 is with template B isgreater than the correlation of template interval 566 with template C.Since the correlation between interval 566 and template B is greaterthan the correlation between interval 566 and template C, the morphologyof interval 566 is identified as having the same morphology as templateB. On the other hand, if the correlation between interval 566 andtemplate B was greater than the correlation between interval 566 andtemplate C, the morphology of interval 566 would be identified as havingthe same morphology as template C.

The process is repeated for the remainder of the twelve intervals, withthe morphology of each interval either being identified with one of thepreviously generated templates, either the highest of multipledetermined correlated templates, or with the single correlated template,or with a new template based on the current interval, so that, returningto FIG. 3, once the final interval 568 is identified with a template,the generated template that is correlated with the greatest number ofintervals (template B in the example of FIG. 7) is set as the templateassociated with the detected event in and is stored in memory Block 326.In addition, the generated template that is correlated with the greatestnumber of intervals must also include a predetermined percentage, suchas 80% for example, of all of the intervals utilized. The event is thenclassified as being a VT/VF event Block 322.

If the number of atrial sensed events A_(S) occurring during the window400 is greater than the number of ventricular sensed events V_(S), andtherefore the A/V ratio is neither equal to one, No in Block 306, norless than one, No in Block 324, a determination is made as to whetherthe data utilized by the device during the initial classification of theevent as a detected event includes intervals that are regular, Block328.

FIG. 8 is a graphical representation of exemplary data utilized in theclassification of an event by an implantable medical device. In theexemplary graphical representation of FIG. 8, the number of intervals todetection (NID) is 16 intervals. Therefore, during the initial detectionprocess, the device 220 has detected the occurrence of a cardiac eventonce the detection criteria have been met, i.e., once 16 intervalshaving a rate less than the threshold rate are detected. According to anembodiment of the present invention, in order to determine whether thedata utilized by the device during the initial classification of theevent as a detected event includes intervals that are regular, a modesumof the 16 RR intervals resulting in detection is generated bydetermining whether the number of intervals in the two highest modes(i.e., most frequent bins) is greater than a predetermined percentage ofthe number of RR intervals, such as 67% for example. In particular, inthe exemplary data illustrated in FIG. 8, since the two most frequentbins both include eight of the sixteen intervals at interval lengths of330 ms and 340 ms, the modesum is 100% and therefore greater than thepredetermined percentage of the number of RR intervals. Therefore, theRR intervals are determined to be regular, Yes in Block 328.

If the RR intervals are determined not to be regular, No in Block 328,the event is identified as being unable to be evaluated and the processcontinues with a next post-processing determination of the accuracy ofthe classification of the event, such as Block 332 for example,described below. On the other hand, if the RR intervals are determinedto be regular, Yes in Block 328, a determination is made as to whetherthe A/V intervals associated with the initial identification of theevent as a cardiac event are stable, Block 330. For example, accordingto an embodiment of the present invention, in order to determine whetherthe A/V intervals associated with the initial identification of theevent as a cardiac event are stable, PR intervals, i.e., the timebetween an atrial sense 800 and a subsequent ventricular sense 802, aredetermined for each of the intervals associated with the initialidentification of the event as a cardiac event are determined.

In order to reduce the effect of outliers, once the PR intervals aredetermined for each of the intervals associated with the initialidentification of the event as a cardiac event, a predetermined numbermaximum PR intervals and the minimum PR intervals are removed. Forexample, according to an embodiment of the present invention, one sixthof the maximum PR intervals and one sixth of the minimum PR intervalsare removed. A PR range is then determined as the difference between theminimum PR interval and the maximum PR interval, and a determination ismade as to whether a range of the PR intervals satisfies a PR stablecriteria. For example, for the data of FIG. 8, a determination is madeas to whether the range of the remaining PR intervals is less than 20ms.

In the example of FIG. 8, once the minimum and the maximum intervalswere removed, the PR range is 10 ms, and therefore the range of the PRintervals is determined to less than 20 ms.

If the range of the PR intervals is determined to less than or equal to20 ms and therefore the A/V intervals associated with the initialidentification of the event as a cardiac event are stable, Yes in Block330, the event is re-classified as being a supraventricular event, Block310. On the other hand, if the range of the PR intervals is determinedto be greater than 20 ms and therefore the A/V intervals associated withthe initial identification of the event as a cardiac event are notstable, No in Block 330, the minimum interval and the maximum intervalof the NID intervals associated with the detection of the event areidentified and a determination is made as to whether the differencebetween the maximum interval and the minimum interval is greater than apredetermined threshold, such as 100 ms for example, Block 332.

If the difference between the maximum interval and the minimum intervalis not greater than a predetermined threshold, No in Block 322, theevent is identified as being unable to be evaluated and the processcontinues with a next post-processing determination of the accuracy ofthe classification of the event, such as Block 336, described below. Ifthe difference between the maximum interval and the minimum interval isgreater than the predetermined threshold, Yes in Block 332, themorphology of the maximum interval is compared with the morphology ofthe minimum interval, and a determination is made as to whether thecorrelation of the maximum interval with the minimum interval is greaterthan a predetermined correlation threshold, Block 334, such as 0.94 forexample.

If the correlation of the maximum interval with the minimum interval isnot greater than the predetermined correlation threshold, No in Block334, the detected event is re-classified as being a supraventriculartachycardia, Block 310. If the correlation of the maximum interval withthe minimum interval is greater than the predetermined correlationthreshold, Yes in Block 334, the morphologies of each of the intervalsassociated with the detecting of the event are compared with thetemplate or templates stored in Block 326, and a determination is madefor each as to whether the correlation of the morphology is greater thana predetermined correlation threshold, such as 0.95 for example. Adetermination is made as to whether the number of intervals thatcorrelate with the stored threshold is greater than a predeterminedmatching percentage threshold, such as 80% for example.

If the number of intervals having morphologies 80 that are determined tohave a correlation with the stored template that is greater than thecorrelation threshold is greater than the predetermined threshold, Yesin Block 336, the previously detected event is re-classified as being aventricular tachycardia event, Block 338. If the number of intervalshaving morphologies 80 that are determined to have a correlation withthe stored template that is greater than the correlation threshold isnot greater than the predetermined threshold, No in Block 336, are-classification of the detected event is not possible, and thereforethe previously detected event is identified as being unknown, Block 340.

In this way, by enabling review and post-processing of each detectedevent, the present invention reduces the time required by a clinician orother medical personnel to review the accuracy of classification ofepisodes by the device in order to identify inappropriate detections,and also decreases the likelihood that time is spent reviewingdetections unnecessarily. In addition, as less experienced medicalpersonal review the episodes stored in the memory of an implantablemedical device, the process of the present application will provide anincreased likelihood that a correct classification will be made.

FIG. 10 is a graphical illustration of determining whether atrial sensedevents are evenly distributed with the ventricular sensed eventsaccording to an embodiment of the present invention. As illustrated inFIG. 10, according to an embodiment of the present invention, during thedetermination as to whether the atrial sensed events A_(S) are evenlydistributed with the ventricular sensed events V_(S) in Block 308,

FIG. 9 is a flowchart of a method of determining whether data previouslyutilized by a device to identify a cardiac event corresponds to regularsensing of events by the device according to an embodiment of thepresent invention. As illustrated in FIG. 9, according to an embodimentof the present invention, during the determination as to whether theatrial sensing is both spontaneous and regular in Block 302, retrievedstored data associated with prior classifications of events by thedevice 220 is reviewed by the access device 240 to identify whetherthere are instances where oversensing is suspected to have occurred,Block 900. For example, a determination is made by the access device 240as to whether oversensing is suspected to have occurred in Block 900 bydetermining whether there are instances where the device 220 hasidentified the existence of far-field R-waves (FFRWs) that have beenidentified by the device 220 as atrial sensed events, using knowndetection algorithms, such as those described in U.S. Pat. No. 6,178,350to Gillberg et al., U.S. Pat. No. 6,052,620 to Gillberg et al., and U.S.Pat. No. 5,755,736 to Gillberg et al., for example, incorporated hereinby reference in their entireties. The R-waves that were identified bythe device 220 as far-field R-waves are then classified by the accessdevice 240 as being suspected far-field R-waves.

In addition, other factors that are utilized by the access device 240 toidentify suspected far-field R-waves in Block 900 may include thoseinstances of far-field R-waves that are located after the period ofdetection by the device 220, instances of simultaneous atrial andventricular events, except in instances where the ventricular event isTP (Ventricular Pace for ATP), and instances where a predeterminednumber of PP intervals, such as four for example, are determined to bestable when a suspected far-field R-wave(s) in that period of true PPintervals are ignored. According to an embodiment of the presentinvention, PP intervals are also considered to be stable if each valueof the predetermined number of PP intervals is within a predeterminedrange, such as less than or equal to 110% and greater than or equal to91%, for example.

Once the suspected far-field R-waves have been identified, Yes in Block900, using the techniques described above, all of the determinedsuspected far-field senses are removed, Block 902, and the intervals arere-calculated without the suspected far-field R-wave, Block 904. Adetermination is then made as to whether the signal having the suspectedfar-field R-waves removed is regular, Block 906. According to anembodiment of the present invention, in order to determine whether thesignal having the suspected far-field R-waves removed is regular, amodesum of the signals starting with and working backward in time fromthe interval corresponding to detection of the event by the device 220is generated by the access device 240, and a determination is made as towhether the modesum is greater than or equal to 99%.

If the signal having the suspected far-field R-waves removed isdetermined to be regular, i.e., the modesum is greater than or equal to99%, Yes in Block 906, the signal is determined to be spontaneous andregular, Yes in Block 302 of FIG. 3, and the process continues with thedetermination of the A/V ratio, Block 304 of FIG. 3 as described above.If the signal having the suspected far-field R-waves removed isdetermined to not be regular, i.e., the modesum is not greater than orequal to 99%, No in Block 906, a determination is made as to whether thedata received from the device 220 by access device 240 includes anatrial electrogram, Block 908. If the data received from the device 220by access device 240 does not include an atrial electrogram, No in Block908, such information is stored by the access device 240, and the signalis determined to be spontaneous and regular, Yes in Block 302 of FIG. 3,and the process continues with the determination of the A/V ratio, Block304 of FIG. 3 as described above.

If the data received from the device 220 by access device 240 doesinclude an atrial electrogram, Yes in Block 908, the suspected far-fieldR-waves that were removed in Block 902 are inserted back within thesignal, Block 910, and the first suspected far-field R-wave prior to thepoint of detection of the event by the device 220 is identified, Block912. A window of sensed beats is then formed about the suspectedfar-field R-wave, Block 914, using a predetermined number of beatssubsequent to and prior to the far-field R-wave. For example, accordingto an embodiment of the present invention, the window of Block 914includes 10 sensed beats, including six beats prior to the suspectedfar-field R-wave and three beats subsequent to the suspected far-fieldR-wave (i.e., the far-field R-wave is the seventh of ten beats). Oncethe window is determined in Block 914, a template is generatedcorresponding to the morphology of each of the intervals in the window,Block 916, and a determination is made as to whether the morphology ofthe suspected far-field R-wave has a predetermined correlation, such as0.92 for example, with an interval in the window that is a non-suspectedand non-confirmed beat, Block 918.

If the morphology of the current suspected far-field R-wave isdetermined to have the predetermined correlation with an interval in thewindow that is a non-suspected and non-confirmed beat, Yes in Block 918,the current suspected far-field R-wave is classified as not being afar-field R-wave, Block 920. If the morphology of the current suspectedfar-field R-wave is determined not to have the predetermined correlationwith an interval in the window that is a non-suspected and non-confirmedbeat, No in Block 918, a determination is made as to whether the currentsuspected far-field R-wave has a predetermined correlation, such as 0.82for example, with a predetermined number of previously confirmedfar-field R-waves, such as 67% for example, Block 922.

If the current suspected far-field R-wave has the predeterminedcorrelation with the predetermined number of previously confirmedfar-field R-waves, Yes in Block 922, the current suspected far-fieldR-wave is classified as a confirmed far-field R-wave, Block 926. Ifthere are no previously confirmed far-field R-waves or if the currentsuspected far-field R-wave does not has the predetermined correlationwith the predetermined number of previously confirmed far-field R-waves,No in Block 922, a determination is made as to whether the currentsuspected far-field R-wave has the predetermined correlation with apredetermined number of true p-waves in the window (i.e., intervals thatare neither suspected nor confirmed far-field R-waves), such as 67% forexample, Block 924.

If the current suspected far-field R-waves has the predeterminedcorrelation with the predetermined number of true p-waves, Yes in Block924, the current suspected far-field R-wave is classified as not being afar-field R-wave, Block 920. However, if the current suspected far-fieldR-waves does not has the predetermined correlation with thepredetermined number of true p-waves, No in Block 924, the currentsuspected far-field R-wave is classified as a confirmed far-fieldR-wave, Block 926.

Once the current suspected far-field R-wave is classified as a confirmedfar-field R-wave, Block 926 or as not being a far-field R-wave, Block920, a determination is made as to whether all of the suspectedfar-field R-waves have been classified by the access device 240, Block928. If all of the suspected far-field R-waves have not been classifiedby the access device 240, No in Block 928, the next suspected far-fieldR-wave is located, Block 930, the window is determined as describedabove in Block 914 for the next far-field R-wave and the process isrepeated. If all of the suspected far-field R-waves have been classifiedby the access device 240, Yes in Block 928, the classified far-fieldR-waves are removed from the signal, Block 932, the signal is determinedto be spontaneous and regular, Yes in Block 302 of FIG. 3, and theprocess continues with the determination of the A/V ratio, Block 304 ofFIG. 3 as described above.

According to an embodiment of the present invention, once the currentsuspected far-field R-wave is classified as a confirmed far-fieldR-wave, Block 926, this classification made be further verified byremoving the classified far-field R-wave, re-calculating the resultingnew A-A interval and determining whether the new A-A interval is lessthan an average of a predetermined number of previous A-A intervals byless than or equal to a predetermined threshold. For example, adetermination is made as to whether the new A-A interval is 30 ms orless than an average of ten previous A-A intervals. If the new A-Ainterval is 30 ms or less than the average of ten previous A-Aintervals, the classification of the interval as a confirmed far-fieldR-wave is confirmed. However, if the new A-A interval is not 30 ms orless than the average of ten previous A-A intervals, the classificationis changed from being a confirmed far-field R-wave to being not afar-field R-wave. The process then continues by determining whether allof the suspected far-field R-waves have been classified by the accessdevice 240, Block 928.

As illustrated in FIG. 9, if no suspected far-field R-waves wereidentified by the device 220 and therefore the access device 240determines that oversensing is not suspected to have occurred, No inBlock 900, a determination is made by the access device 240 as towhether undersensing has occurred, Block A. FIG. 10 is a flowchart of amethod of determining whether data previously utilized by a device toidentify a cardiac event corresponds to regular sensing of events by thedevice according to an embodiment of the present invention. Asillustrated in FIG. 10, according to an embodiment of the presentinvention, the determination as to whether undersensing has occurred ismade, for example, by determining whether one of a predetermined numberof undersensing criteria have been met, Block 950. An examples of anundersensing criteria would include determining whether at least onesensed AA interval associated with predetermined beats, such as the NIDventricular beats prior to detection of the event and the atrialinterval immediately subsequent to the detection of the event forexample, greater than a predetermined interval, such as 2500 ms forexample. Another example of an undersensing criteria would includedetermining whether the atrial channel includes less than apredetermined number of events, such as nine events for example, priorto detection.

Another example of an undersensing criteria according to the presentinvention would include calculating a median atrial interval in asliding window of a predetermined number of intervals, such as eightintervals for example, and determining whether the median atrialinterval is greater than a predetermined threshold, such as 1300 ms forexample. If the median atrial interval is greater than the predeterminedthreshold, atrial undersensing is determined to have occurred, Yes inBlock 950. Atrial undersensing is also determined to have occurred ifthe median is greater than a predetermined threshold, such as 350 ms forexample and a current atrial interval, such as the final atrial event inthe window for example, i.e., the eighth event, is greater than themedian by a predetermined threshold, such as 1.7 times the medianinterval or more, for example. Atrial undersensing is also determined tohave occurred if the median is within a predetermined range, such asgreater than 200 ms and less than or equal to 359 ms for example, and acurrent atrial interval, such as the final atrial event in the windowfor example, i.e., the eighth event, is greater than the median by apredetermined threshold, such as 1.9 times the median interval or more,for example. Atrial undersensing may be determined to have occurred ifthe median is greater than 0 ms and less than or equal to 200 ms, and acurrent atrial interval, such as the final atrial event in the windowfor example, i.e., the eighth event, is greater than the median by apredetermined threshold, such as 3.25 times the median interval or more,for example.

Another criteria for determining whether atrial undersensing hasoccurred, according to an embodiment of the present invention, includesdetermining whether there are two or less intervals in the entire recordthat are two times the median AA interval, or three times the median AAinterval and a majority of the AA intervals are regular, and there is adepolarization on the atrial egm signal where the expected atrial eventwould have occurred. FIG. 11 is a graphical illustration of determiningwhether undersensing has occurred according to an embodiment of thepresent invention. For example, as illustrated in FIG. 11, regular AAintervals (approximately equal to 300 ms) are sensed for a majority ofthe sensed atrial events A_(S), such as 92% for example, and there isonly one or two depolarizations that are either an interval 965 that is900 ms and therefore three times the median AA interval (300 ms), or aninterval 967 that is 600 ms and therefore two times the median AAinterval, and for each interval 965 and 967 there is an associateddepolarization, 969 and 971 respectively, on the atrial EGM.

A final exemplary criteria for determining atrial undersensing hasoccurred includes determining whether a predetermined number of the AAintervals in the entire record are within a predetermined range of themedian of the AA intervals, such as 92% within the range of the median,and there are less than or equal to four long intervals, i.e., two orthree times the median interval, when the EGM is not stored.

As illustrated in FIG. 10, if one of the undersensing criteria are notmet, No in Block 950, the signal is determined to be spontaneous andregular, Yes in Block 302 of FIG. 3, and the process continues with thedetermination of the A/V ratio, Block 304 of FIG. 3 as described above.If one of the undersensing criteria are met, Yes in Block 950 and if theundersensing is repairable, the location of the undersensed event isdetermined, Block 952. The undersensing is repairable if the median AAinterval is between 300 and 750 ms (inclusive), and if the intervalcontaining the undersensed event is a multiple of 2 or 3 times themedian AA interval. If undersensing is not repairable, such as in thecases where the atrial sensed interval is >2500 or there are only 9atrial events prior to detection, then undersensing is reported but notrepaired.

According to an embodiment of the present invention, if the interval 967is two times the median AA interval, the location of the undersensedevent is determined by inserting a single sensed atrial event at themidpoint of the interval 967. If the interval 965 is three times themedian AA interval, the location of the undersensed event is determinedby inserting a first sensed atrial event at a distance corresponding tothe median AA interval from a starting point 973 of the interval, and asecond interval at a distance corresponding to the median AA intervalprior to an end point 975 of the interval 965.

Some of the techniques described above may be embodied as acomputer-readable medium comprising instructions for a programmableprocessor such as microprocessor 142, pacer/device timing circuit 178 orcontrol circuit 144 shown in FIG. 3. The programmable processor mayinclude one or more individual processors, which may act independentlyor in concert. A “computer-readable medium” includes but is not limitedto any type of computer memory such as floppy disks, conventional harddisks, CR-ROMS, Flash ROMS, nonvolatile ROMS, RAM and a magnetic oroptical storage medium. The medium may include instructions for causinga processor to perform any of the features described above forinitiating a session of the escape rate variation according to thepresent invention.

While a particular embodiment of the present invention has been shownand described, modifications may be made. It is therefore intended inthe appended claims to cover all such changes and modifications, whichfall within the true spirit and scope of the invention.

1. A method of determining undersensing during post-processing ofsensing data generated by a medical device, comprising: transmitting aplurality of stored sensing data generated by the medical device to anaccess device, the stored sensing data including sensed atrial eventsand sensed ventricular events; determining, in response to thetransmitted data, instances where the medical device identified acardiac event being detected in response to the sensing data; anddetermining whether one of a predetermined number of undersensingcriteria have been met in response to the transmitted data.
 2. Themethod of claim 1, wherein the predetermined number of undersensingcriteria comprises determining whether at least one sensed AA intervalassociated with predetermined beats, are greater than a predeterminedinterval.
 3. The method of claim 1, wherein the predetermined number ofundersensing criteria comprises determining whether an atrial channelincludes less than a predetermined number of events prior to detection.4. The method of claim 1, wherein the predetermined number ofundersensing comprises: calculating a median atrial interval in asliding window of a predetermined number of intervals; and determiningwhether the median atrial interval is greater than a predeterminedthreshold.
 5. The method of claim 4, wherein the predetermined number ofundersensing further comprises determining whether the median is greaterthan a predetermined threshold, and a current atrial interval is greaterthan the median by a predetermined threshold.
 6. The method of claim 1,wherein the predetermined number of undersensing comprises determiningwhether the median is within a predetermined range, and a current atrialinterval is greater than the median by a predetermined threshold.
 7. Themethod of claim 1, wherein the predetermined number of undersensingcomprises: determining whether there are two or less intervals in thetransmitted data that are one of two times a median AA interval;determining whether there are three times the median AA interval;determining whether a majority of the AA intervals are regular; anddetermining whether there is a depolarization on a corresponding atrialegm signal where a corresponding expected atrial event would haveoccurred.
 8. The method of claim 1, wherein the predetermined number ofundersensing comprises: determining whether regular AA intervals aresensed for a majority of sensed atrial events; and determining whetherthere is only one or two depolarizations that one of three times themedian AA interval and two times the median AA interval and acorresponding associated depolarization on an atrial EGM.
 9. The methodof claim 1, wherein the predetermined number of undersensing comprises:determining whether a predetermined number of the AA intervals in thetransmitted data within a predetermined range of a median of the AAintervals; and determining, in response to an atrial EGM not beingincluded in the transmitted data, whether there are less than apredetermined number of intervals that are one of two and three timesthe median interval.
 10. A system for post-processing of sensing dataassociated with identification of a cardiac event, comprising: a medicaldevice generating and storing a plurality of sensing data, the storedsensing data including sensed atrial events and sensed ventricularevents; an access device located externally from the medical device; andan interface for transmitting the stored sensing data from the medicaldevice to the access device, wherein the access device determineswhether one of a predetermined number of undersensing criteria have beenmet in response to the transmitted data.
 11. The system of claim 10,wherein the access device determines whether at least one sensed AAinterval associated with predetermined beats, are greater than apredetermined interval.
 12. The system of claim 10, wherein the accessdevice determines whether an atrial channel includes less than apredetermined number of events prior to detection.
 13. The system ofclaim 10, wherein the access device calculates a median atrial intervalin a sliding window of a predetermined number of intervals, anddetermines whether the median atrial interval is greater than apredetermined threshold.
 14. The system of claim 13, wherein the accessdevice determines whether the median is greater than a predeterminedthreshold, and a current atrial interval is greater than the median by apredetermined threshold.
 15. The system of claim 10, wherein the accessdevice determines whether the median is within a predetermined range,and whether a current atrial interval is greater than the median by apredetermined threshold.
 16. The system of claim 10, wherein the accessdevice determines whether there are two or less intervals in thetransmitted data that are one of two times a median AA interval, whetherthere are three times the median AA interval, whether a majority of theAA intervals are regular, and whether there is a depolarization on acorresponding atrial EGM signal where a corresponding expected atrialevent would have occurred.
 17. The system of claim 10, wherein theaccess device determines whether regular AA intervals are sensed for amajority of sensed atrial events, and whether there is only one or twodepolarizations that one of three times the median AA interval and twotimes the median AA interval and a corresponding associateddepolarization on an atrial EGM.
 18. The system of claim 10, wherein theaccess device determines whether a predetermined number of the AAintervals in the transmitted data within a predetermined range of amedian of the AA intervals, and determines, in response to an atrial EGMnot being included in the transmitted data, whether there are less thana predetermined number of intervals that are one of two and three timesthe median interval.